In mobile communication devices such as a PDC, a PHS, etc., an audio or speech signal is converted into digital data on the transmitting side and a carrier is modulated by the digital data and transmitted by radio. On the receiving side, the digital data is demodulated from the received modulated signal and the original audio signal is reproduced from the demodulated digital data (see, for example, "The Sep. 12, 1994 issue of Nikkei Electronics (no. 617)" published by Nikkei BP Co., Ltd., pp 71-96 &lt;Special Edition: Mounting Technology of Portable Telephones Competing with one another in Less Weight and Low Price&gt;).
As a digital data modulating system, a kind of phase modulation (PSK modulation) is mostly utilized which associates the value of each digital data with a displacement or shift in the phase of a carrier. In QPSK modulation, four types of phase shifts (0, .pi./2, .pi., 3.pi./2) corresponding to four kinds of binary logical data of "00", "01", "10", "11" are effected on the carrier. On the demodulating side, the phase shift in the carrier is detected for each predetermined interval and binary logical data associated with the detected phase shift is reproduced.
In digital communications, an amplitude phase modulation system is also used which causes both the phase and amplitude of a carrier to hold information for effective use of the frequency. As a system for demodulating the amplitude phase-modulated signal, a method of orthogonally detecting a received signal and thereafter identifying a phase component and amplitude thereof, a method of identifying a phase component by orthogonal or rectangular detection and identifying an amplitude component by envelope detection, etc. are known (see Japanese Patent Application Laid-Open No. 6-244888 and the like).
The aforementioned process for demodulating the phase-modulated signal has heretofore been executed using analog circuits such as a lowpass filter, a delay detector, etc., whereas the process for demodulating the amplitude phase-modulated signal has heretofore been performed using analog circuits such as a local oscillator for orthogonal detection, a multiplier, an envelop detector, etc. Since, however, the analog circuits are hard to be brought into semiconductor integration as compared with digital circuits, and the effect of reducing a chip area by process miniaturization and the stability of performance are also deteriorated, some or all of the analog circuits have been replaced with digital circuits in recent years. The replacement of the analog circuits with the digital circuits allows an improvement in the degree of integration of a semiconductor integrated circuit. Thus, a reduction in the number of parts, less size and weight of a device, the suitability for mass production, and the stability and reproducibility of performance, etc. are to be expected.
It has however been definitely shown by the present inventor et al. that the aforementioned technique involves the following problems.
That is, the function of the conventional analog circuit has been replaced by that of the digital circuit as it is in the phase demodulator using the conventional digital circuits. The demodulation processing system itself still remains in the realm of simple digital simulation of the operation of each analog circuit. Described specifically, an A/D converter is placed on the input side and a phase-modulated signal digitized (quantized) by the A/D converter is electrically processed by a digitized lowpass filter and a digitized delay detector or the like. Thus, the degree of integration of the semiconductor integrated circuit can be enhanced even by the circuit replacement that the digital circuit simply follows the system using the analog circuits as it is.
However, the digital process for simulating the operation of the analog circuit as it is makes it to increase the amount of data to be handled. Further, the processing of the data referred to above also results in an increase in the number of manhours and high complexity. As the A/D converter used for digitizing the phase-modulated signal, one is needed which is as fast as possible in speed and high in resolution. However, the high-speed and high-resolution A/D converter encounters many difficulties in its manufacturing and is extremely high in cost.
Digitizing the phase demodulator makes it so effective in enhancing the degree of integration of the semiconductor integrated circuit. However, it was not possible to avoid an increase in the scale and cost of a circuit with its digitization and a reduction in processing speed due to an increase in the quantity of data and complexity of its processing by any means.
An object of the present invention is to provide a technique capable of implementing a process for digitally demodulating a phase-modulated signal, in a simple and small-scale configuration suitable for semiconductor integration and a reduction in cost and in a configuration easy to provide speed-up and high accuracy without using a high-speed and high-resolution A/D converter.
Another object of the present invention is to provide a technique capable of realizing a process for digitally demodulating an amplitude phase-modulated signal by digital circuits having only the minimum analog circuits required and principal parts suitable for semiconductor integration, without using a rectangular detection circuit, an envelop detection circuit and a high-speed and high-resolution A/D converter.
The above and other objects and features of the present invention will become apparent from the description of the present specification and the accompanying drawings.